Known image forming apparatus comprising a photosensitive drum and adapted to form an image by electronic photography are required to keep the photosensitive drum electrified so as to uniformly show a certain electric potential. For this purpose, an electric potential measuring instrument is used to measure the electric potential of the electrified photosensitive drum and a feedback control system is operated to keep the photosensitive drum electrified so as to uniformly show a predetermined electric potential, utilizing the reading of the instrument.
Now, the operation of a known electric potential measuring instrument will be discussed below. A technique of mechanically modulating an electric field and inducing an alternating current to electrodes is often used for non-contact type electric potential measuring instruments (mechanical AC field induction type). With this technique, the electric potential of the surface of the object of measurement is a function of the magnitude of the electric current i that is taken out from a detection electrode contained in the electric potential measuring instrument as expressed by formula (1) below;i=dQ/dt=d(C·V)/dt  (1),where Q is the quantity of the electric charge that appears on the detection electrode, C is the coupling capacitance between the detection electrode and the object of measurement and V is the surface voltage of the object of the measurement.
The capacitance C is expressed by formula (2) below;C=A·S/x  (2),where A is a constant of proportion, S is the surface area of the detection electrode and x is the distance between the detection electrode and the object of measurement.
The quantity of the electric charge that appears on the detection electrode is very small and can easily be influenced by the noise found around it. For this reason, a synchronous detection method is often used to measure the very small quantity of charge Q. In other words, a necessary signal is obtained by periodically modulating the magnitude of the capacitance C between the detection electrode and the object of measurement by an appropriate means and detecting same frequency components from the observed signal.
In the case of the above described typical method, a fork-shaped shutter is inserted between the object of measurement and the detection electrode and periodically moved in a direction running in parallel with the surface of the object of measurement to partly block the lines of electric force arriving at the detection electrode from the object of measurement in a mode of periodically modulating them so as to effectively change the area S of the detection electrode and consequently modulate the electrostatic capacitance C between the object of measurement and the detection electrode (U.S. Pat. No. 4,720,682).
On the other hand, there is an increasing demand for small photosensitive drums to be used in electrophotographic image forming apparatus and related components are required to be arranged highly densely around the drum. At the same time, electric potential measuring instruments are also required to be downsized and made thin. However, in the case of above described currently available mechanical AC field induction type sensors, the internal space of the sensor structure is mostly occupied by the drive mechanism for driving the fork-shaped shutter, or the fork-shaped oscillator to oscillate, and some other assembly components. Therefore, the drive mechanism needs to be indispensably downsized in order to downsize the electric potential measuring instrument.
In view of the above identified technical circumstances, the magnitude of the electric current that is taken out from a mechanical AC field induction type electric potential sensor is given by formula (3) below on the basis of the above formulas (1) and (2).i=d(A·V·S/x)/dt  (3)
Therefore, as the electric potential sensor is downsized, the surface area S of the electrode is reduced to in turn reduce the output current i of the sensor. Such a sensor is disadvantageous in terms of accuracy of measurement because it is highly apt to be influenced by external noise.